Despite widespread attempts to increase physical activity levels in the general population, only 32% of U.S. adults regularly participate in recommended levels of either moderate or vigorous physical activity. Workplaces are important settings for physical activity programs, but the effectiveness of such programs for increasing physical activity has not yet been determined by a theoretically based, randomized controlled trial. The study we propose will involve 1,600 male and female employees recruited from 16 work sites of The Home Depot, Inc. The purpose of the study will be to determine the effects of a multi-level intervention aimed at personal goal setting by employees and ecologically based organizational action. Worksites will be paired according to size and workplace characteristics, and each site will be randomly assigned to either the intervention or control conditions. The primary outcome measure will be moderate to vigorous physical activity. Secondary outcomes will include stage of physical activity change; self-rated productivity, healthrelated quality of life, measures of workplace morale; and social-cognitive mediators of physical activity change derived from goal setting theory, social-cognitive theory, and stage theory. Statistical analysis of the intervention effects will be performed using hierarchical linear modeling. We hypothesize that the intervention will increase moderate to vigorous physical activity and the secondary outcomes. Innovative and unique features of the study will be: [1] implementation of a multi-level intervention that targets (a) the workplace culture using socio-ecologically based organizational action and (b) employee motivation using peer-group leadership of goal-based cognitive-behavior modification, aided by the use of low cost pedometers, and [2] assessment of the effects of the intervention on mediators of change in the outcome measures using structural equation modeling. The project will implement and rigorously evaluate a multilevel physical activity intervention that has potential for broad application to workplace health promotion.